In order to improve performance of a fuel injection valve in an internal combustion engine, it is necessary to assess spray characteristics such as a diameter distribution of fuel liquid drops contained in the spray and separation states of liquid and gas phases. As methods of counting the number of ultrafine particles suspended in air, there is known a method including coarsening fine particles through condensation under oversaturated atmosphere and optically counting the number of coarsened fine particles (for example, see Patent Literature 1). In addition, there is Patent Literature 2 as a prior art reference in relation to the present invention.